I. Field of the Invention
This invention relates generally to implantable cardiac rhythm management devices used in medical treatment. More particularly, the present invention relates to a method and apparatus for determining whether a patient suffering from congestive heart failure (CHF) will benefit from pacing therapy through an implantable cardiac rhythm management device.
II. Description of the Prior Art
CHF is a disease that affects over two million Americans. Patients suffering from this disease typically manifest a degraded cardiac function, such as abnormally low cardiac output. The average life span of a patient diagnosed with CHF is just five years.
Implantable cardiac rhythm management devices, when they were first introduced, were simply used to deliver ventricular stimulating pulses to maintain a predetermined heart beat rate. More recently, as discussed in a paper entitled "Usefulness of physiologic dual-chamber pacing in drug-resistant idiopathic dilated cardiomyopathy" in the American Journal of Cardiology (Vol 66, Jul. 15, 1990, pp. 198-202), implantable cardiac rhythm management devices have been used to enhance the hemodynamic performance of the heart in patients suffering from CHF or other types of left ventricular dysfunction. It has been found that by pacing the left ventricle and appropriately setting the atrio-ventricular (AV) delay parameter of a pacemaker, the patient's cardiac output can be improved and optimized. The following patents include methods for optimizing the AV delay interval and/or pacing mode when treating CHF patients: the Salo U.S. Pat. No. 5,312,452; the Salo et al. U.S. Pat. Nos. 5,344,222 and 5,487,752; the Spinelli et al. U.S. Pat. No. 5,466,245; the Tockman U.S. Pat. No. 5,540,727; and the Baumann U.S. Pat. No. 5,800,471.
Pacing therapy has been found to have both positive and negative acute impact on CHF patients. In effect, some CHF patients respond well to the pacing therapy, and are called "responders", while others are negatively effected, and are called "non-responders". It is speculated that the different responses of CHF patients to pacing therapy are due to different patterns and/or degrees of conduction abnormalities in the hearts of the patients.
One study that has been published by Circulation (1999; 99:2993-3001) demonstrates the different responses of CHF patients to pacing therapy. Twenty-seven patients with New York Heart Association (NYHA) Class III/IV heart failure, and QRS duration greater than or equal to 120 msec, were enrolled in the "Pacing Therapies for Congestive Heart Failure" (PATH-CHF) study. Twenty-five of the patients were ultimately tested to see the effect of pacing chamber and AV delay on acute systolic function. These patients were paced at right (RV), left (LV) and both ventricles (BV) at one of five AV delays. Each combination of pacing chamber and AV delay was randomly repeated five times on each patient in a five-beat pacing/fifteen-beat no-pacing duty cycle.
The results of the test showed that pacing significantly increased both the LV pressure derivative maximum and pulse pressure for fifteen (15) of the patients (60%), increased only the LV pressure derivative maximum for five (5) of the patients (20%), and did not increase the LV pressure derivative maximum or pulse pressure for another five (5), which was 20%, of the patients. The group of five patients that did not have an increase in either their LV pressure derivative or pulse pressures were called "non-responders". The rest of the patients responded well to pacing and were considered to be "responders".
The PATH-CHF study revealed a trend with respect to the responders and non-responders. The study revealed that the responders had wider QRS complexes, as recorded on surface electrocardiograms (ECG), than the non-responders. Wider than normal QRS complexes indicate a lack of synchrony in ventricular depolarizations and contractions, which may be attributable to CHF or some other conduction abnormality. All but two of the responder patients had QRS complex widths greater than 150 msec, while all of the non-responder patients had QRS complex widths less than 150 msec. It appears that there is a trend showing that the patients with the worst conditions are more likely to benefit from pacing therapy.
A problem with the QRS width method of assessing "responsiveness" is that the margin of separation between those patients determined to be responders and non-responders is less than 10%. This narrow margin makes the method inaccurate. Therefore, a need exists for a patient identification method to separate the responders from the non-responders with a wide margin of separation. After considering the prior art relating to this invention, it can be seen that a need exists for an easy and accurate method and apparatus for determining whether a CHF patient is properly suited for pacing therapy. The present invention meets this need.